Diagenesis And Its Influence To The Pore Of The Lower Cretaceous Reservoir In Tamtsag Basin In The East Of Mongolia

The study area of this paper is southern Tamtsag depression in Tamtsag Basin in the east of Mongolia,the study stratigraphic horizon are Lower Cretaceous Tongbomiao Formation,Nantun Formation and Damoguaihe Formation.In the fields of diagenesis study,there is few prevenient research on diagenesis of the volcano-sedimentary rocks.By means of ordinary slice and cast sections observations,SEM experiment,electronic probe analysis,porosity analysis and examination,the types and distribution of the reservoir rocks,the types and characteristics of diagenesis as well its influence to the pore can be investigated.1.Rock TypesIn accordance with the sequence of the weakening of volcanism and the strengthening of sedimentation process,the rocks in this area can be classified into five types include lava, pyroclastic, sedimentary volcanicclastic rocks, volcaniclastic sedimentary rocks and normal sedimentary rocks. Normal sedimentary rock and volcaniclastic sedimentary rocks accounted for the main part.2.Diagenesis StudyIt is shown that the diagenetic types developed in this area include compaction and pressure-solution,cementation,fusion sintering and corrosion,dissolution, replacement,the formation of neogenic minerals, recrystallization, devitrification and so on.The typical diagenetic sequence is as follows:welding and corrosion,the start of mild compaction and pressolution,tuffaceous cementation—ferruginous cementation—alteration of tuffaceous,feldspar and rock debris—athougenic kaolinite, weak overgrowth quartz or microcrystalline quartz—transformation of clay minerals,appearance of illite smectite mixed layer and illite, chlorite cementation,clay minerals cladding—strengthening of pressolution,stronger overgrowth quartz and Siliceous cementation—stronger calcite cementation and replacement—appearance of dawsonite—dissolution of calcite and the formation of a large number of secondary porosity—appearance of autogenic feldspars—appearance of laumontite—appearance of epidote.The stage of diagenesis is among Phase A in late diagenetic stage.3. Analysis of Pore CharacteristicsThe mainly types of pore developed in this area are intergranular dissolution pore and tuffaceous fillings intragranular pore.Compaction and cementation decrease the pore of reservoir,dissolution increase the pore of reservoir.4. Prodict for Advantage Pore Developed ZoneThrough the porosity analysis,between the depth of 1850-2100m,there is an abnormal high porosity zone in Tanan depression.This zone distributed widely in the southwest well areas and also partially in the northeast in Tanan depression.